Defense Department officials say that No Easy Day, former Navy SEAL Matt Bissonnette's book about the secret mission to kill Osama bin Laden, includes classified information that may harm U.S. military operations. The book went on sale yesterday despite the Pentagon's warnings of possible legal action last week.

Referring to the book, which was written under the pseudonym Mark Owen, Pentagon press secretary George Little today called it "the height of irresponsibility not to have this kind of material checked for the possible disclosure of classified information."

Speaking with reporters, Little called it "a no-brainer" that a book about a clandestine operation like the one that killed bin Laden would need to be reviewed by the Pentagon. He later added that Defense officials "are very concerned that — that classified information may be contained in this book."

Asked repeatedly for clarification, Little went a bit further, saying that "sensitive and classified information is contained in the book."

But Little also said that the Pentagon would not attempt to block the sale of the book, either to the American public or on the bookshelves of military exchanges.

The discussion came a day after the leader of the Naval Special Warfare Command, Rear Adm. Sean Pybus wrote a letter telling the service members in his group that "hawking details about a mission" and other details about SEAL activities could put them all at risk.

"For an elite force that should be humble and disciplined for life, we are certainly not appearing to be so," Pybus wrote in the letter, which was obtained by the Associated Press. "We owe our chain of command much better than this."

As of Tuesday afternoon, the book was in the No. 1 spot on Amazon's bestseller list. It had garnered four stars, on the strength of 34 reviews. But some of the reviews were contentious, as folks took stands over whether Bissonnette should have written the book. One negative review attracted more than 70 comments — yes, more than twice the amount of people who wrote about the book itself.